A bar code is a computer-readable code which represents information. One type of well-known bar code employs a series of parallel bars for encoding a bit string. Other types of bar codes include bar codes formatted as two dimensional matrices.
Bar codes may be read by bar code scanners. One well-known type of bar code scanner generally includes a laser light source, a photodetector, an amplifier, a filter and processing circuitry. In general terms, this type of bar code scanner operates by sweeping a laser beam across a bar code. The beam is swept, for example, by a rotating mirror such as that disclosed in U.S. Pat. No. 5,331,140 (Stephany), which is incorporated herein by reference for its teaching of means for sweeping a beam. Alternatively, one of the other known equivalent means for sweeping a beam may be employed. The light which is received by the photodetector during sweeping of the beam is analyzed to determine the bit string encoded in the bar code.
One well-known type of bar code, which will be referred to herein as a "standard" bar code, is comprised of dark and light bars of different widths. When a laser beam is swept across this type of bar code, the beam is mostly absorbed by the dark parts of the bar code and mostly scattered by the light parts of the bar code. Consequently, during the sweep, the photodetector receives some of the light from the laser beam which is scattered when the beam is directed toward the light parts of the bar code, and receives a relatively negligible amount of light from the laser beam when the beam is directed toward the dark parts of the bar code. The pattern of light received by the photodetector accordingly corresponds to the pattern of bars of the bar code.
The photodetector produces an analog signal which represents the light received during the sweep of the laser beam. This signal is provided to an amplifier which amplifies the signal. The amplified signal is provided to a filter, typically an analog band pass filter that is tuned to reduce primarily those components of the amplified electric signal which do not correspond to the scattered beam. The filtered signal then is provided to the processing circuitry (which can include a combination of analog and digital circuitry), which processes the signal to determine the bit string represented by the bar code.
A second, less common, type of bar code is printed using fluorescent ink. When the fluorescent ink is irradiated by light having a wavelength which is the excitation wavelength of the ink, the ink emits light within a known band of wavelengths which does not include the first wavelength. The fluorescent ink is generally only minimally visible to the human eye under normal lighting conditions. Depending upon the type of ink employed, the light emitted by the fluorescent ink may also be minimally visible or invisible to the human eye.
When fluorescent ink is used to print a bar code, the bar code is read by sweeping the bar code with a laser having a wavelength which is the excitation wavelength of the fluorescent ink. When the beam is swept across the bar code, the bar code emits light from the bars which are printed with the fluorescent ink, and does not emit light from the bars which are represented as blanks (not printed). The light received by the photodetector accordingly includes light scattered by the substrate and light emitted by the fluorescent ink, and therefore varies in amplitude with the pattern of light emitted by the fluorescent bar code.
Fluorescent bar codes have certain advantages over standard bar codes. For example, standard bar codes generally require a plain background which does not interfere with the bar code, and therefore generally must either be segregated from other printed matter or obstruct it. In contrast, a fluorescent bar code can placed anywhere on an object without altering the appearance of the object itself. This is useful for consumer products such as magazines where the visual appearance of the packaging is important. Fluorescent bar codes may also be placed on an object without obstructing other critical information on the object, such as the written address on a piece of mail.
One notable difference between standard and fluorescent bar codes is that they use opposite printing standards for representing "high" and "low" bits of the bar code. In a standard bar code, a light bar represents a high bit, while a dark bar (or printed bar, since standard bar codes are generally printed on relatively light surfaces) represents a low bit. Conversely, in a fluorescent bar code, a high bit is represented by a fluorescent (printed) bar, while a low bit is represented by an unprinted (and therefore non-fluorescent) bar.
There is presently one manner known in the art for reading both fluorescent and standard bar codes using a single reader. U.S. Pat. No. 5,502,304 (Berson et al.) discloses a bar code reader that is capable of reading a standard bar code and a bar code printed in fluorescent ink. The Berson system employs a standard laser beam for reading a standard bar code and an ultraviolet light source for exciting a fluorescent bar code to fluorescence as it is being scanned. The system further employs two separate receivers for receiving laser light scattered by the standard bar code and for receiving light emitted from the fluorescent bar code.